200 



THE ANATOMY OF WOODY PLANTS 



secondary growth. In the Lepidodendraceae in the narrower sense 

 the secondary wood is distinguished from the primary struc- 

 tures only by the presence, exclusively, of radial parenchyma 

 and by the radial seriation of its tracheary elements (Fig. 185). 

 The organization of the tracheids of the secondary wood is, in fact, 

 identical with that found in the primary region. In the higher 

 lepidodendroid forms assembled under the appellation Sigillariae 



very frequently, 

 particularly in the 

 region of the 

 secondary wood 

 more remote from 

 the pith, the tra- 

 cheids cease to be 

 scalariform and 

 assume the pitted 

 type characteristic 

 of the gymno- 

 sperms and other 

 higher representa- 

 tives of the Vascu- 

 lares. 



The vascular 

 strands of leaves in 

 the lepidodendrids 

 are characterized, 



as are those of the Lycopodiales in general, by mesarch organiza- 

 tion. This condition is clearly shown in Fig. 186. It is appar- 

 ent that the foliar trace is surrounded by secondary wood. 

 It has been stated by Scott that true transfusion tissue is present 

 in the leaf of the lepidodendrids, but this statement, in view of the 

 situation present in the Lycopodiales in general and in the Lepido- 

 dendraceae in particular, seems open to some question, and certainly 

 the subject seems to require further investigation. The foliar organs 

 of the lepidodendroid stock were characterized by the presence of 

 two aerating strands on either side of the foliar trace; these were 

 in communication below with the external air through the agency 



FIG. 185. Longitudinal section through primary and 

 secondary wood of Lepidodendron species. 



